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Post-harvest rind pitting studies on ‘Valencia’ orange

Ehlers, Jacques Louis (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Post-harvest rind pitting is a non-chilling related physiological rind disorder that affects various citrus cultivars and reduces fruit value. This disorder is characterised by the collapse of the flavedo sub-epidermal cells, however the main cause of this disorder is unknown, but it is aggravated by changes in relative humidity (RH) and rind water status. Studies were conducted on ‘Turkey’ and more susceptible ‘Benny’ valencia oranges in Limpopo and Mpumalanga South Africa. The effect of fruit position, maturity and size on fruit susceptibility to this disorder was investigated and it was found that fruit from the outside of the canopy are more susceptible to this disorder probably due to greater exposure to variation in environmental conditions than fruit from the inside of the canopy. More mature fruit were also found to be slightly more susceptible, however size did not influence incidence of this disorder. Various plant growth regulators were also evaluated to prevent pitting. The application of the synthetic auxins 2,4-dichlorophenoxy acetic acid (2,4-D) and 3,5,6 trichloro-2-pyridiloxyacetic acid (3,5,6 TPA) at 50 % petal drop (2,4-D) or after physiological fruit drop (2,4-D or 3,5,6-TPA) reduced the incidence of post-harvest pitting. Application of s-abscisic acid 1 week before harvest was also found to reduce incidence of this disorder, however gibberellic acid applied in January did not reduce the incidence of post-harvest rind pitting. A systemic fungicide thiabendazole (TBZ) which reduces the incidence of chilling injury also reduced post-harvest pitting incidence if applied before fruit were subjected to stress inducing environmental conditions. Pre-harvest foliar application of TBZ 1 week before harvest and post-harvest dip treatments directly after harvest reduced post-harvest weight loss and incidence of this disorder. A citrus industry survey was conducted to estimate the financial impact of this disorder at foreign and local markets on producers. Markets generating higher prices had a lower tolerance for incidence of post-harvest rind pitting than lower priced markets and are therefore seen as high-risk. Due to this large reduction in market price for fruit with the disorder, treatments found during this study might be cost effective.